This invention relates generally to semiconductor laser diode structures and, more particularly, to superluminescent diodes. By way of background, a semiconductor laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forward-biased in normal operation, electrons and holes combine in the region of the active layer, and light is emitted. The layers on each side of the active layer have a lower index of refraction than the active layer, and function as cladding layers to confine the light in the plane of the active layer. Various techniques are used to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction in the structure. If the diode current is above a threshold value, lasing takes place and light is emitted from one of the facets, in the plane of the active layer.
The output of a laser diode is generally monochromatic and coherent, and results from the stimulated emission of light from the active layer and the repeated reflections between the laser facets. For some applications, coherent light is not required and a broader spectrum of radiation is obtained. When a diode structure is configured to produce spontaneous, but not stimulated, light emission over a broad range of wavelengths, it is referred to as a superluminescent diode, or SLD. High power SLDs with wide spectral bandwidths are needed for a number of applications, such as fiber-optic gyroscopes. There is also a need in some applications for SLDs of narrower bandwidth, but still not operating as lasers.
The difficulty with designing an SLD is to devise a technique that will effectively suppress lasing. Because the structure typically has to include end facets for the emission of light, there is a natural tendency for light to be repeatedly reflected between the facets, and for the device to stabilize its operation in one or a small number of longitudinal modes of oscillation. Existing designs for SLDs typically employ antireflective coatings on the end facets, to suppress repeated reflections through the gain region of the device. Other designs have suggested non-uniformities in the active layer to inhibit lasing. However, as a practical matter it is very difficult to design and fabricate a high-power SLD that will not inadvertently revert to lasing operation. The present invention overcomes this difficulty.